Expose error_code so consumers can read a code instead of parsing one

io_error_code was private, so the predicates built on it (is_halt,
is_skippable_title_stub, is_disc_level_no_key) were the only way to ask
anything about an io::Error's origin. A consumer that needs the code
itself — to report WHY a title failed rather than to branch on one of
three known cases — had no route to it: mux_stream returns an io::Error,
the typed Error is gone by then, and only the E<code> string prefix
survives.

That left every front-end to re-implement the prefix parse by hand,
which is precisely the string-matching 1.5.x spent its time removing.
One parser, exported.

No behaviour change: the function is unchanged and the three predicates
still call it.

src/io/writeback_file/macos.rs

@@ -217,7 +217,7 @@ mod tests {
 
         // The three arms must be DISTINGUISHABLE, not merely non-Ok. Each
         // carries its own numeric code through the "E<code>" prefix that
-        // `From<Error> for io::Error` mints — the only shape `io_error_code`
+        // `From<Error> for io::Error` mints — the only shape `error_code`
         // recognises. A bare `ErrorKind` cannot be classified, which is how a
         // user cancel here used to read as a hard I/O failure.
         let lost = bounded_failure_to_result(BoundedError::WorkerLost)